Bias in Clinical Trials

1. Where Bias Can Be Introduced

1.2. Information (or Measurement) Bias

This type of bias refers to a systematic error in the measurement of participant characteristics, exposure or treatment outcome, for example:
  • Participants may be wrongly classified as exposed to being at higher risk of disease when they are not. 
  • The disease may be reported as progressing when it is stable.
This might be due to:
  • Inaccuracies in the measurement tool. 
  • Expectations of trial participants – a participant could be more optimistic because they are assigned to the new treatment group .
  • Expectations of the investigators – also called the ‘observer bias’. This occurs when:
    • Investigators are very optimistic about the effect of the new treatment and interpret more favourably any clinical signal.
    • Investigators monitor the adverse effects of the new treatment more carefully than for the standard treatment. 

Blinding  the allocated treatment to the participant and/or the investigators may prevent such biasBlinding is of special interest when the trial outcome is subjective, like the reduction in pain, or when an experimental treatment is being compared to a placebo. However, while a blinded randomised trial is considered the gold standard of clinical trials, blinding may not always be feasible: 

  • Treatments may cause specific adverse effects that make them easy to identify.

  • Treatments may need different procedures for administration or different treatment schedules.